Nested coreference, i.e. a mention with a coreferring mention within its own span, may seem odd, but is in fact rather frequent, and attested in this very sentence. ON does allow nested coreference, but excludes cases without subsequent mention, leading to another difficult learning task for systems (the non-markable status of such cases hinges on later sentences), and unresolved pronouns in text, as in REF , which remains unannotated in OntoNotes, presumably because outside of the larger phrase's boundaries, it appears to be a singleton.In fact, some systems have relied on this constraint, sometimes called `i-within-i' due to the co-index \(i\) , to rule out certain kinds of match candidates, see [1]}.
